TFE-ethylene copolymer is excellent in chemical resistance, and electrical and mechanical properties. Further, it is moldable by injection or extrusion molding in the same way as usual thermoplastic resins.
However, the TFE-ethylene copolymer tends to be cracked at a high temperature. In order to overcome the defect of the TFE-ethylene copolymer, it is proposed to copolymerize the third monomer as a modifier. For example, Japanese Patent Publication No. 12109/1964 discloses the TFE-ethylene copolymer which contains isobutylene as the modifier to lower the molding temperature so that discoloration of the copolymer at a high temperature is prevented. The publication does not refer to the crack resistance of the copolymer at a high temperature, but in fact, its crack-resistance is not improved.
Japanese Patent Publication No. 23671/1972 discloses copolymerization of a vinyl monomer having a side chain having at least two carbon atoms as the third monomer with TFE and ethylene to improve the mechanical properties of the TFE-ethylene copolymer at a high temperature. Examples of such vinyl monomer are vinyl compounds of the formulas: R--CF.dbd.CF.sub.2, RO--CF.dbd.CF.sub.2 and CH.sub.2 .dbd.CX--CH.sub.2 --R wherein R is a C.sub.2 -C.sub.8 hydrocarbon group, and X is hydrogen or methyl. Among the vinyl compounds, the former two compounds have poor copolymerizability with TFE and/or ethylene and are expensive and hardly obtainable, which makes the copolymer uneconomical. The TFE-ethylene copolymer containing the last vinyl compound as the third monomer has comparatively poor heat resistance.
As a result of the extensive study to improve the drawbacks of the conventional copolymers comprising TFE and ethylene, it has now been found that the copolymerization of 2-trifluoromethylpropylene or 2-methyl-1,1,3,3,3-pentafluoropropylene as the third monomer with TFE and ethylene improve the stress crack resistance of the copolymer at a high temperature, and that the polymerizability of the third monomer is better than the above-described vinyl compounds.